Sonar (SOund NAvigation Ranging) technology is used to detect objects under the water. A sonar device emits acoustic pulses in water and receives an echo from any objects that the acoustic pulse reflects back from. The distance between the sonar device and the object can be determined by measuring the time between the pulse transmission and reflected pulse reception. Active sonar creates a pulse of sound, often called a “ping”, and then listens for reflections of the pulse. To measure the distance to an object, one measures the time from emission of a pulse to reception. The acoustic pulse travels at the speed of sound underwater, thus the distance is determined by the (speed of sound)/(time between sending and receiving the pulse/2).
The pulse may be at constant frequency or a chirp of changing frequency. For a chirp, the receiver correlates the frequency of the reflections to the known chirp. The resultant processing gain allows the receiver to derive the same information as if a much shorter pulse of the same total energy were emitted.